1. Field of the Invention
The present invention relates to a ferroelectric memory, for example, to a crosspoint ferroelectric memory.
2. Background Art
A ferroelectric memory is a semiconductor memory including a ferroelectric capacitor as a component of a memory cell. In general, the memory cell of the ferroelectric memory includes the ferroelectric capacitor and a transistor. In a current semiconductor processing technique, transistors having the same characteristics can be manufactured only on a surface of silicon. Therefore, the bit number of the ferroelectric memory cannot exceed “the chip area divided by the transistor area”, even if the size of the ferroelectric capacitor is minimized to its limit. This is the same in other semiconductor memories including transistors as components of memory cells.
An example of a structure for overcoming this restriction is a memory structure called a crosspoint type. For example, in a crosspoint resistor memory, a memory cell includes a resistor, but does not include a transistor. Therefore, the crosspoint resistor memory is advantageous in that it has a high probability of realizing a highly integrated memory. This is the same in other crosspoint memories.
However, the crosspoint memory has a problem in that a write current leaks also around cells other than a write target cell. Accordingly, the crosspoint memory is defective in that it has a low cell selectivity. Therefore, in the crosspoint memory, a diode is ordinarily inserted to the memory cell to improve the cell selectivity.
However, when this method is applied to the ferroelectric memory, a problem arises in that a current to the capacitor can flow only in one direction due to a rectification by the diode. This is not preferred because the ferroelectric memory must reverse the direction of the current flow to the capacitor, depending on whether it writes “0” or “1” in the memory cell. Accordingly, the ferroelectric memory to which the above method is applied is disadvantageous in that it can write only one of “0” and “1”. In this way, the crosspoint ferroelectric memory has a problem in that it is difficult to secure the selectivity of the memory cell.
Japanese Patent Publication No. 3327071 discloses an example of a ferroelectric memory device in which a memory cell includes only a ferroelectric capacitor. Japanese Patent Publication No. 3327071 further discloses an example of a setting of voltages which are applied to a selected cell and a non-selected cell.